System for live audio presentations

ABSTRACT

In a live audio presentation system, a stage rack and a front of house rack are interconnected by a transmission medium that transmits digital audio data and control information between them. Packets of digital audio data and control information are sent at the audio sampling rate. The packets of data are encoded using an encoding technique that provides unique codes that may be detected, thus enabling clock recovery of a clock signal embedded in the data packets. Data packets may include data for error checking. A serial digital transmission medium may be used as the transmission medium. Such a transmission medium uses low voltage signals and supports high bit rates. The system is synchronized by the FOH rack which transmits data to the stage rack according to a local audio sample clock or an external audio sample clock to which it synchronizes. This clock is embedded in data packets sent to the stage rack. The stack rack recovers the audio sample clock from data received from the FOH rack and synchronizes its receive and transmit operations to this recovered clock. The FOH rack recovers the audio sample clock from data received from the stage rack and synchronizes its receive operations to this recovered clock. The FOH rack may include an embedded computer that processes the digital audio data, and may include DSP processing that handles so-called plug-in software. A digital audio workstation or sequencer also may be connected to the FOH rack to either record the live presentation or to inject edited, recorded audio into the live presentation.

CROSS REFERENCE TO RELATED APPLICATION

This application is a nonprovisional application that claims the benefitunder 35 U.S.C. 119 of prior provisional patent application Ser. No.60/609,084, filed Sep. 10, 2004.

BACKGROUND

In live audio presentations, equipment such as microphones, speakers,amplifiers, instruments and other audio devices have inputs and outputsthat are connected, whether through wired or wireless connections, to acentral location called a stage rack. The stage rack commonly includessignificant audio processing. A console (often referred to as the frontof house) is connected to the stage rack to provide controls for mixing,level controls and other operations. The audio processing is generallylimited to the capabilities of the stage rack and/or the front of houseconsole.

The connection between the stage rack and the front of house consoletypically is a cable carrying analog signals. The reliability of thecabling is important so that the live audio presentation is notadversely affected. Most data loss in such environments is due tocatastrophic failures, typically of the cabling or connectors for thecabling. Data loss between the stage rack and front of house due tonoise or interference occurs less frequently in a properly installedsystem.

Digital interconnects between the stage rack and the front of houseconsole have improved audio performance over analog interconnects, withreduced noise and distortion. However, current digital interconnectshave their own drawbacks. For example, a (MADI) interconnect requiresseparate synchronization and control streams. Ethernet and other networkbased connections tend to have high latency and some protocols do notguarantee data delivery. Digital interconnects are still subject to dataloss due to catastrophic failures of the cabling or connectors.

SUMMARY

In a live audio presentation system, a stage rack and a front of houserack are interconnected by a transmission medium that transmits digitalaudio data and control information between them. Packets of digitalaudio data and control information are sent at the audio sampling rate.The packets of data are encoded using an encoding technique thatprovides unique codes that may be detected, thus enabling clock recoveryof a clock signal embedded in the data stream. The encoding techniquealso adds bits of data to the data stream, which improves thereliability of transmission of the data stream. Data packets may includedata for error checking, and error checking and correction, such as acyclical redundancy check field, and Reed-Solomon, Turbo Code, or othererror correcting codes. A serial digital transmission medium, such ascommonly used for serial video data transmission, may be used as thetransmission medium. Such a transmission medium uses low voltage signalsand supports high bit rates.

The system is synchronized by the FOH rack which transmits data to thestage rack according to a local audio sample clock or an external audiosample clock to which it synchronizes. This clock is embedded in thedata stream sent to the stage rack. The stage rack recovers the audiosample clock from data received from the FOH rack and synchronizes itsreceive and transmit operations to this recovered clock. The FOH rackrecovers the audio sample clock from data received from the stage rackand synchronizes its receive operations to this recovered clock.Redundant transmission media can be used to connect the stage rack andthe FOH rack. If one of the transmission media fails, the receiver caninstantly, automatically and transparently transition to processingsignals received from the other transmission medium.

The synchronization to the external source may be enabled in a mannersuch that the external source is locked to only if that signal is withina specified target range of a sampling rate. If this signal is out oftarget range, locking to this signal is not enabled. If locking isenabled to this signal and this signal drops, or rises, out of targetrange, but within a wider tracking range, locking to this external clockis maintained. If locking is enabled to this signal and this signaldrops, or rises, out of tracking range, locking to this external clockis disabled. Allowing the signal to track outside the range ensuresgreater reliability during a performance during which a reference clockmay drift.

The FOH rack may route digital audio data and control information to aconsole for processing by the console. The FOH rack also, oralternatively, may include an embedded computer that processes thedigital audio data using control signals from the console or from a userinterface on the FOH rack. The processing performed at the FOH rack orconsole may include DSP processing, such as may be found in digitalaudio workstations or sequencers. The embedded computer also may sendcontrol signals to the console to indicate various status information,e.g., meters and fader positions.

A failure of the embedded computer needs to be isolated from theprocessing performed by the DSPs so as to permit live audio processingto continue even if the embedded computer of the FOH rack fails and isrestarted. Separate reset domains are provided in the hardware for theDSPs, control surface and embedded computer. Also, a direct connectionbetween the DSPs and the control surface is provided, for example over acontroller area network (CAN) bus. The embedded computer also connectsto the DSPs, for example over a PCI bus. When the embedded computer isoperational, control information passes between the control surface andthe DSPs over the CAN bus. The DSPs provided information to the computerwhich parses the information and provides control changes to the DSPs.The computer also determines what control information should be sent tothe control surface. If the computer fails, the DSP take over theresponsibility of interpreting data received from the control surfaceand for determining which control information to send to the controlsurface. A restart of the embedded computer does not terminateprocessing by the DSPs and permits the control surface to exchange datadirectly with the DSPs while the embedded computer is restarting.

The DSPs also may execute audio processing functions that are defined byso-called plug-in software. The use of such plug-ins provides theoperator with the ability to add software-based audio processingfeatures, executed on the DSP, thus providing flexibility to expand thecapabilities of the FOH rack. However, such plug-ins also includegraphical user interface processes that are executed on the embeddedcomputer in the FOH rack or console. A failure of the plug-in's processon an embedded computer could result in a failure on the embeddedcomputer. To reduce the possibility of such failures, plug-ins areexecuted in a separate process from the main application running on theembedded computer that handles communication of audio and controlinformation among the main application, the DSPs and the console. Inthis manner, its failure does not terminate operation of other processeson the computer. However, the failure of this process on the computerwill not terminate processing by the DSPs. The main audio workstationapplication may be provided with “patch bay” software to permit theoperator to bypass the operations performed by the failed plug-in on theDSPs.

A digital audio workstation or sequencer also may be connected to theFOH rack to either record the live presentation or to inject edited,recorded audio into the live presentation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagram of an example live audio presentation system.

FIG. 2 is a block diagram of an example implementation of an interfacebetween a stage rack and a front of house (FOH) rack.

DETAILED DESCRIPTION

Referring to FIG. 1, an overview of an example live audio presentationsystem will now be provided. One or more stage racks 100 are provided toconnect to various inputs and outputs, such as microphones, speakers,amplifiers, instruments and other audio devices (not shown) that may beused on a stage in a live presentation. The stage rack(s) 100 connect toa front of house (FOH) rack 102, typically located at the front of anauditorium or the like, and thus at a distance from the stage. Anoperator uses a console 104 connected to the FOH rack 102 to controlprocessing performed on digital audio data. An FOH rack may merely routedigital audio data and control information to the console for processingby the console. The FOH rack also, or alternatively, may include anembedded computer, such as a general purpose computer configured with anembedded operating system, such as WINDOWS XP EMBEDDED, that runs anaudio processing application that processes the digital audio data usingcontrol signals from the console. A control surface also may be one ofthe inputs to the stage rack and may connect to the stage rack through acontroller area network (CAN). Signals from this control surface alsocould be used to control the audio processing performed at the FOH rackor console. The embedded computer also may send control signals to theconsole to indicate various status information, e.g., meters and faderpositions.

The processing performed at the FOH rack may include DSP processing thathandles so-called plug-in software as described in U.S. Pat. No.5,842,014, hereby incorporated by reference. Such a system distributesprocessing tasks among one or more general purpose DSPs or othersuitable processors. The distribution of tasks is performed by anintermediate level software object or “plug-in.” The plug-in is designedto allocate resources of all or part of one or more DSP chips for aspecific, limited set of signal processing algorithms associated withthe plug-in. The DSP code running on the DSP chips is dedicated to theplug-in and is written to efficiently implement the set of algorithmsassociated with the plug-in. As a user of the plug-in requests aspecific algorithm that is one of those algorithms among its set, theobject allocates DSP resources to implement that algorithm in the mostefficient way it is able. The flexibility of performing such processingis enhanced by connecting the processors in a time division multiplexed(TDM) bus structure.

A failure of the embedded computer needs to be isolated from theprocessing performed by the DSPs so as to permit live audio processingto continue even if the embedded computer of the FOH rack fails and isrestarted. Separate reset domains are provided in the hardware for theDSPs, control surface and embedded computer. Also, a direct connectionbetween the DSPs and the control surface is provided, for example over acontroller area network (CAN) bus. Any control information from acontroller connected to the stage rack also can be sent by the FOH rackto the DSPs over the CAN bus. The embedded computer also connects to theDSPs, for example over a PCI bus. When the embedded computer isoperational, control information passes between the control surface andthe DSPs over the CAN bus. The DSPs provided information to the computerwhich parses the information and provides control changes to the DSPs.The computer also determines what control information should be sent tothe control surface. If the computer fails, the DSP take over theresponsibility of interpreting data received from the control surfaceand for determining which control information to send to the controlsurface. A restart of the embedded computer does not terminateprocessing by the DSPs and permits the control surface to exchange datadirectly with the DSPs while the embedded computer is restarting.

Because the plug-ins also include graphical user interfaces that areexecuted on the embedded computer in the FOH rack or console, a failureof the plug-in process on the embedded computer could result in afailure on the embedded computer. To reduce the possibility of such afailure, plug-ins are executed in a separate process from the mainapplication running on the embedded computer that handles communicationof audio and control information among the main application, the DSPsand the console. In this manner, its failure does not terminateoperation of other processes on the computer. However, the failure ofthis process on the computer will not terminate processing by the DSPs.The main audio workstation application may be provided with “patch bay”software to permit the operator to bypass the operations performed bythe failed plug-in on the DSPs or to mute the operations of the failedplug-in.

If the plug-in process fails, it also is possible to rebuild the plug-inprocess, while audio continues to be processed. First, if all of theDSPs are bypassed and muted, the DSPs hosting the plug-ins is reset. Anew process hosting the plug-ins is created, and then the plug-ins arereinstantiated in both the embedded computer and on the DSP. After theprocess and DSPs are restarted with the plug-ins, the user mayre-inserted them into the live audio processing. In other words, thegrouping of the plug-ins in a process on the embedded computer, and on areserved set of DSPs on the DSPs, allows for protection of the DSPs,should the embedded computer's process hosting plug-ins fail. The audiomixing engine is capable of isolating the DSP processing of theplug-ins, so that even after the process hosting the plug-ins hasfailed, each plug-in can be bypassed or muted. After bypassing or mutingall the (DSP) Plug-Ins, a new process hosting plug-ins can be recreatedwithout affecting the audio mixing of the system. The plug-ins can berecreated in that process and on the DSP(s) reserved for plug-ins. Thenat the user's discretion, the DSP Plug-In effects can be re-inserted inthe audio processing, as they were before the initial problem occurred.

Additional reliability is provided by dividing the main audioapplication into other separate processes. In particular, a separateprocess for handling communication with the control surface is executedas a separate process. A separate data model process handles monitoringthe internal state of the console and communication with the DSPs. Yetanother separate process executes the graphical user interface for themain audio application through the operating system resources. Finally aseparate shell process monitors the status of the other process andattempts to relaunch them if they fail or crash.

A digital audio workstation or sequencer 106 also may be connected tothe FOH rack 102 to either record the live presentation or to injectedited, recorded audio into the live presentation.

The connection between the stage rack 100 and the FOH rack 102 isillustrated at 108 and comprises a transmission medium, e.g., a coaxialcable, twisted pair, fiber optic link, etc., for each direction ofcommunication. That is, there is a transmission medium for sendingdigital data, including digital audio data and control information, fromthe stage rack to the FOH rack, and another transmission medium forsending such digital data from the FOH rack to the stage rack. Thetransmission medium in each direction of communication also may beredundant, with transitioning from one medium to its backup medium whena failure is detected. The transition can be made without perceptibledata loss. Thus, in the event of a catastrophic failure of onetransmission medium, the backup medium may be used.

Referring now to FIG. 2, a block diagram of an example implementation ofthe interconnection between the stage rack and the FOH rack will now bedescribed.

The transmission media 208 and 212 may be implemented according to thespecifications for transmission media for standard serial digitalinterfaces for video signals, particularly SMPTE-292M (used for highdefinition video) and SMPTE-259M (used for standard definition video).Such transmission media are useful for transmitting digital dataincluding digital audio data and control information because they uselow voltage (one volt peak-to-peak) signals and support high data rates.Other transmission media also may be used, such as fiber optic cable.

The data sent over the transmission medium may be encoded using 8B10Bencoding. Such encoding facilitates galvanic isolation (by usingtransformers and/or capacitors) because it uses a DC-free signalpattern. Such encoding also uses and a set of special codes that areunique, i.e., no random combination of other codes can generate apattern that includes one of the special codes. By having such specialcodes, one of the special codes, e.g., the K28.5 code in 8B10B encoding,may be used as a top of frame indicator, from which a clock signal canbe recovered. Other encoding techniques with similar characteristicsalso may be used, such as 4B5B, 16B20B, or biphase mark encoding. ErrorDetecting and Correcting Codes such as Cyclic Redundancy Check (CRC),Reed-Solomon, Turbo-Code, or other codes may be used for error detectionand error correction.

On the stage rack side 200, logic 202 provides audio sample clocks. Theaudio sample clock may be, for example, 48 kHz, in which case the wordclock may be 12.288 MHz and the bit clock may be 122.88 MHz, assuming256 words of data to be transmitted per audio sample clock period, and10 bits per word. Logic 202 also connects to an input/output interface204 through which input audio data and control information and outputaudio data and control information may be communicated to other devicesin or connected to the stage rack. Such an interface may provide data toand receive data from an interface such as described in U.S. patentapplication Ser. No. 10/342,963, filed Jan. 15, 2003 and entitled “Audioand Music Data Transmission Medium and Transmission Protocol.”

The logic 202 includes registers for storing control information andbuffers for storing input audio data and output audio data. Data in thebuffers and the control information are packaged into data packets thatare encoded and transmitted to the FOH rack. How the control informationand buffers are arranged and processed into packets depends on theimplementation. The data packets include a cyclic redundancy check (CRC)data field to include a CRC code generated by the logic. Data packetsreceived from the FOH rack are buffered, decoded and separated into dataand control information. The CRC data in a received packet is used toconfirm that the data has been reliably received.

As an example implementation, each data packet may be designed to betransmitted in a single frame, within one period of the audio sampleclock signal. The data packet includes a top of frame code (provided byone of the special codes of the encoding scheme, e.g. the K28.5 code for8B10B), followed by several words, such as one to eight words, ofcontrol information. The audio data appears next in the packet. Forexample, 72 channels of 24-bit encoded data (resulting in 30-bits perchannel when using 8B10B encoding) may be provided. This encoded datamay include audio data for several channels and additional control data.A cyclic redundancy check field follows, and null fields may be used aspadding in the data packet to force it to fit a frame. The ordering andpositions of the audio channels, and the number and positions of wordsof control information in a data packet depends on the implementation.It also is possible to provide some control information after the audiodata instead of null packets. Each data packet may originate in logic202 as a sequence of 8-bit words, but is subsequently encoded, if using8B10B encoding, into a sequence of 10-bit words for transmission. Inthis implementation, 256 words of data, and thus 2560 bits, are used perpacket in transmission. Thus, if packets are transmitted at 48 kHz, thena bit clock of 122.88 MHz is used.

The logic 202 provides 8B10B encoded data to transmit circuitry 206which sends the encoded data to the FOH rack over the transmissionmedium 208. The transmit circuitry transforms the 8B10B encoded datainto a signal suitable for transmission over the transmission medium208. The transmit circuitry 206 uses the recovered clock provided by thereceive circuitry 210 described below. Pre-emphasis may be used in thetransmit circuitry 206 to improve signal quality. If the transmissionmedium includes redundant media, the signal is sent over the redundantmedia as well.

Receive circuitry 210 receives data from the FOH rack over thetransmission medium 212. It includes a clock recovery circuit togenerate a clock signal to extract the data from the received signal.The receive circuitry 210 outputs the extracted 8B10B data to the logic202 along with the recovered clock signal. The receive circuitry 210 mayinclude adaptive equalization to improve signal quality. The clockrecovery circuit preferably uses fast-locking phase-locked loops (PLLs)to improve the speed of changeover from one transmission medium toanother when a failure is detected. An example clock recovery circuitthat may be used is the Micrel SY87700L. Due to the signals on thetransmission medium having the format described above, the data may bereadily extracted using known techniques for processing such signals.

If the transmission medium includes redundant cabling, the receivecircuitry includes a multiplexer that selects one of the receivedsignals for processing. The receive circuitry also includes a linedetection circuit that detects whether a signal is available on eachtransmission medium. The line detection circuit can be used to drive theselection of the multiplexer and to provide a visual indication ofwhether a line is active. Other techniques for detecting errors orfailures on a transmission medium may be used, and may drive switchingof reception of a signal from one transmission medium to the other.

On the FOH rack side 240, logic 242 provides audio sample clocks to theprocessing engine, whether a set of DSP processors in an embeddedcomputer in the FOH rack or in the console. The logic 242 also connectsto an input/output interface 244 through which input audio data andcontrol information and output audio data and control information may becommunicated to other devices in or connected to the FOH rack. Such aninterface may provide data to and receive data from an interface such asdescribed in U.S. patent application Ser. No. 10/342,963, filed Jan. 15,2003 and entitled “Audio and Music Data Transmission Medium andTransmission Protocol.”

The operation of logic 242 at the FOH rack is generally the same as forthe stage rack. In particular it prepares and encodes data packets to besent to the stage rack and decodes and processes data packets receivedfrom the stage rack. However, the FOH rack is configured as a master andthe stage rack is configured as a slave for clock signal processing andsynchronization, as described in more detail below. Thus, in the FOHrack, transmission is based on a local clock (whether locally generatedor synchronized to an external source). The FOH can source and sinkaudio data with the stage rack. Received data is processed in the FOHrack based on the clock signal recovered by the receive circuitry.

The logic 242 provides 8B10B encoded data packets to transmit circuitry246 which sends the encoded data to the stage rack over the transmissionmedium 212. The transmit circuitry transforms the 8B10B encoded datainto a signal suitable for transmission over the transmission medium212. The transmit circuitry 246 uses a clock based on a local oscillatoror an external source. The transmit circuitry 246 may use pre-emphasisto improve signal quality. If the transmission medium includes redundantcabling, the signal is sent over the redundant cabling as well.

Receive circuitry 250 receives data from the stage rack over thetransmission medium 208. It includes a clock recovery circuit togenerate a clock signal to extract data from the received signal. Thereceive circuitry 250 outputs the extracted 8B10B data to the logic 242along with the recovered clock signal. The receive circuitry 250 mayperform adaptive equalization to improve signal quality. The clockrecovery circuit preferably uses fast-locking phase-locked loops (PLLs)to improve the speed of changeover from one transmission medium toanother when a failure is detected. An example clock recovery circuitthat may be used is the Micrel SY87700L. As on the stage rack side, dueto the signals on the transmission medium having the format describedabove, the data may be readily extracted using known techniques forprocessing such signals.

If the transmission medium includes redundant cabling, the receivecircuitry includes a multiplexer that selects one of the receivedsignals for processing. The receive circuitry also includes a linedetection circuit that detects whether a signal is available on eachtransmission medium. The line detection circuit can be used to drive theselection of the multiplexer and to provide a visual indication ofwhether a line is active. Other techniques for detecting errors orfailures on a transmission medium may be used, and may drive switchingof reception of a signal from one transmission medium to the other.

As noted above, the FOH rack acts as a master device and provides clocksignals to the stage rack. More particularly, the FOH rack has a localoscillator 256 at the audio sample rate, e.g., 48 kHz, from which theword clock and bit clock signals are generated at 260. These clocksignals are used to control the transmission side of the FOH rack. Whendata is transmitted to the stage rack, each data packet includes aspecial code that occurs in the data stream at the audio sampling rate.The clock recovery circuit in the receive circuitry of the stage rackrecovers a clock signal. The recovered clock is used by the stage rackfor both receiving and transmitting data. This recovered clock also maybe used by the stage rack to synchronize with yet other devices as asub-master. When data is transmitted back to the FOH rack, each datapacket includes the special code that occurs in the data stream at theaudio sampling rate. The clock recovery circuit in the receive circuitryof the FOH rack recovers a clock signal. The recovered clock is used bythe FOH rack for receiving data.

The word clock and bit clock generator 260 and the logic 242 also mayreceive an external audio sample clock signal. This external signal maybe used to synchronize the FOH rack with external equipment. The logic242 may provide a signal to the word clock and bit clock generator 260to enable it to synchronize either to the local oscillator 256 or tothis external source.

The synchronization to the external source may be enabled by logic 242in a manner such that the external source is used only if that signal iswithin a specified target range (e.g., 10 to 30 parts per million) ofthe target audio sampling rate, such as 48 kHz. If this signal is out ofrange the logic 242 does not enable locking to this signal. If lockingis enabled to this signal and this signal drops out of tracking range(e.g. 30 to 50 parts per million), the logic 242 disables locking tothis signal. To measure whether the external source is within range, thelogic 242 detects an event, i.e., a transition in the external sourcesignal over a period of time. After detecting an event, the logic is ina state in which it is not expecting an event during a subsequent periodof time called a window. If an event occurs in this window, then thereis something wrong with the signal and the system does not lock to it.If no event occurs in this window, the logic transitions to a state inwhich it is expecting an event during a subsequent period of time. If anevent occurs in this window, a counter is incremented and the logictransitions back to the state in which no event is expected. If no eventoccurs during the window in which an event is expected, the signal isrejected. After several such transitions, if the counter reaches athreshold level, the signal is deemed to be acceptable.

Having now described an example embodiment, it should be apparent tothose skilled in the art that the foregoing is merely illustrative andnot limiting, having been presented by way of example only. Numerousmodifications and other embodiments are within the scope of one ofordinary skill in the art and are contemplated as falling within thescope of the invention.

1. A stage rack for a live audio presentation system, comprising: logicfor processing input digital audio data and control information and forproviding output digital audio data and control information; a receiveinterface for receiving a signal from a first transmission medium,wherein the signal includes encoded data packets occurring at an audiosampling rate and including a unique code, and for recovering a bitclock signal using the unique codes in the data packets, and forrecovering, and providing to the logic, digital audio data and controlinformation from the data packets; and a transmit interface for sendinga signal over a second transmission medium, wherein the signal includesencoded data packets occurring at the audio sampling rate, wherein theencoded data packets are received from the logic and include digitalaudio data and control information and a unique code from which a bitclock signal can be recovered, wherein bits of the encoded data packetsare sent in synchronization with the bit clock signal recovered by thereceive interface.
 2. The stage rack of claim 1, wherein the logicencodes the digital audio data and control information using a form ofencoding that results in a DC-free signal.
 3. The stage rack of claim 1,wherein the logic encodes the digital audio data and control informationusing a form of encoding that provides unique codes.
 4. The stage rackof claim 3, wherein the form of encoding is 8B10B encoding.
 5. The stagerack of claim 1, wherein the transmission medium is a serial digitalinterface for video data.
 6. The stage rack of claim 1, wherein theencoded data packets include error recovery data.
 7. The stage rack ofclaim 6, wherein the error recovery data includes cyclic redundancycheck data.
 8. A front of house rack for a live audio presentationsystem, comprising: logic for processing input digital audio data andcontrol information and for providing output digital audio data andcontrol information; a receive interface for receiving a signal from afirst transmission medium, wherein the signal includes encoded datapackets occurring at an audio sampling rate and including a unique code,and for recovering a bit clock signal using the unique codes in the datapackets, and for recovering, and providing to the logic, digital audiodata and control information from the data packets; and a transmitinterface for sending a signal over a second transmission medium,wherein the signal includes encoded data packets occurring at the audiosampling rate, wherein the encoded data packets are received from thelogic and include digital audio data and control information and aunique code from which a bit clock signal can be recovered, wherein bitsof the encoded data packets are sent in synchronization with a bit clocksignal generated in response to a reference audio sample clock signal.9. The front of house rack of claim 8, wherein the logic encodes thedigital audio data and control information using a form of encoding thatresults in a DC-free signal.
 10. The front of house rack of claim 8,wherein the logic encodes the digital audio data and control informationusing a form of encoding that provides unique codes.
 11. The front ofhouse rack of claim 10, wherein the form of encoding is 8B10B encoding.12. The front of house rack of claim 8, wherein the transmission mediumis a serial digital interface for video data.
 13. The front of houserack of claim 8, wherein the encoded data packets include error recoverydata.
 14. The front of house rack of claim 13, wherein the errorrecovery data includes cyclic redundancy check data.
 15. The front ofhouse rack of claim 8, wherein the reference audio sample clock signalis generated by a local oscillator.
 16. The front of house rack of claim8, wherein the reference audio sample clock signal is provided by anexternal reference audio sample clock signal, and wherein the logicdetermines whether a rate of the external reference audio sample clocksignal is within a specified range of an expected audio sample clockrate.
 17. A live audio presentation system, comprising; one or morestage racks; a front of house rack; a first transmission mediumconnected between the stage rack and the front of house rack tocommunicate data from the front of house rack to the stage rack; asecond transmission medium connected between the stage rack and thefront of house rack to communicate data from the stage rack to the frontof house rack; wherein the front of house rack includes a transmitinterface for sending a signal over the first transmission medium,wherein the signal includes encoded data packets occurring at the audiosampling rate, wherein the encoded data packets are received from thelogic and include digital audio data and control information and aunique code from which a bit clock signal can be recovered, wherein bitsof the encoded data packets are sent in synchronization with a bit clocksignal generated in response to a reference audio sample clock signal;wherein at least one of the one or more stage racks includes a receiveinterface for receiving a signal from the first transmission medium,wherein the signal includes encoded data packets occurring at an audiosampling rate and including a unique code, and for recovering a bitclock signal using the unique codes in the data packets, and forrecovering digital audio data and control information from the datapackets; wherein the at least one of the one or more stage racksincludes a transmit interface for sending a signal over the secondtransmission medium, wherein the signal includes encoded data packetsoccurring at the audio sampling rate, wherein the encoded data packetsare received from the logic and include digital audio data and controlinformation and a unique code from which a bit clock signal can berecovered, wherein bits of the encoded data packets are sent insynchronization with the bit clock signal recovered by the receiveinterface of the stage rack; wherein the front of house rack includes areceive interface for receiving a signal from the second transmissionmedium, wherein the signal includes encoded data packets occurring at anaudio sampling rate and including a unique code, and for recovering abit clock signal using the unique codes in the data packets, and forrecovering digital audio data and control information from the datapackets.
 18. The live audio presentation system of claim 17, furthercomprising: a first redundant transmission medium connected between thestage rack and the front of house rack to communicate data from thestage rack to the front of house rack; and a second redundanttransmission medium connected between the stage rack and the front ofhouse rack to communicate data from the front of house rack to the stagerack.
 19. The live audio presentation system of claim 18, including ameans for detecting an error on the first transmission medium and forautomatically switching to receiving from the first redundanttransmission medium.
 20. The live audio presentation system of claim 18,including a means for detecting an error on the second transmissionmedium and for automatically switching to receiving from the secondredundant transmission medium.
 21. A live audio presentation system,comprising; one or more stage racks; a front of house rack; a firsttransmission medium connected between the stage rack and the front ofhouse rack to communicate data from the front of house rack to the stagerack; a second transmission medium connected between the stage rack andthe front of house rack to communicate data from the stage rack to thefront of house rack; a first redundant transmission medium connectedbetween the stage rack and the front of house rack to communicate datafrom the stage rack to the front of house rack; and a second redundanttransmission medium connected between the stage rack and the front ofhouse rack to communicate data from the front of house rack to the stagerack; means for detecting an error on the first transmission medium andfor automatically switching to receiving from the first transmissionmedium; means for detecting an error on the second transmission mediumand for automatically switching to receiving from the secondtransmission medium.
 22. A live audio presentation system, comprising;one or more stage racks; a front of house rack; a transmission mediumconnected between the stage rack and the front of house rack tocommunicate data from the front of house rack to the stage rack; whereinthe front of house rack includes: a computer executing a first processfor processing audio information, a set of digital signal processorsconnected to the computer for performing audio processing on audioinformation according to plug-in code provided by the computer; whereinthe computer executes a second process which is separate from the firstprocess and which provides an interface for an operator to controlparameters of the audio processing performed by the set of digitalsignal processors.
 23. The live audio presentation system of claim 22,wherein the front of house rack further comprises: a console having afirst connection with the set of digital signal processors forcommunicating audio information and control information to the set ofdigital signal processors; and wherein the computer has a connection forcommunicating audio information and control information to the set ofdigital signal processors.
 24. The live audio presentation system ofclaim 23, wherein the console communicates control information to theset of digital signal processors, and wherein, when the computer isoperational, the set of digital signal processors provides the controlinformation to the computer and receives control signals from thecomputer based on the control information provided to the computer, andwherein the console communicates control information to the set ofdigital signal processors, and wherein, when the computer is notoperational, the set of digital signal processors generates controlsignals based on the control information provided by the console. 25.The stage rack of claim 3, wherein the encoded data include ForwardError Correction Coding.
 26. The stage rack of claim 25, wherein theForward Error Correction Coding is Turbo-Code.
 27. The stage rack ofclaim 25, wherein the Forward Error Correction Coding is Reed-SolomonCode.
 28. The front of house rack of claim 8, wherein the encoded datainclude Forward Error Correction Coding.
 29. The front of house rack ofclaim 28, wherein the Forward Error Correction Coding is Turbo-Code. 30.The front of house rack of claim 28, wherein the Forward ErrorCorrection Coding is Reed-Solomon Code.
 31. The front of house rack ofclaim 16, wherein the specified range is a target range and the logicalso allows tracking over a wider tracking range after locking to thattarget range.
 32. The live audio presentation system of claim 22,further comprising a control surface connected to the stage rack so asto provide control information from the control surface to the front ofhouse rack.